1. Field of the Invention
The present invention relates to active phased array antennas; and more particularly, to the calibration of the active phased arrays.
2. Description of Related Art
An active phased array antenna is an array of elements that are switched between transmit and receive channels. The plurality of the individual elements are commonly connected to form a transmit/receive module. In the transmit mode a power amplifier activates each module; and in the receive mode, a low noise amplifier amplifies the incoming signal. The array of modules is steered by varying the phase and amplitude of the elements. Current active phased array antennas typically distribute the radar transmit and receive channels among multiple transmit/receive (T/R) modules. These modules require phase and amplitude control not only for steering, but also, to adjust for their own mutual differences and to compensate for any residual errors in the radiators. Since the modules are considerably more active in an active phased array when compared with prior systems employing phase shifters alone, they are prone to drifts in amplitude and phase; and must be continually re-tuned after initial range calibration. Several techniques have been proposed to re-tune the array. As far as is known, they all involve setting up a calibration loop around the T/R modules. A coupler at the output of each module is then attached to a test manifold. One system proposes utilizing a near field source on the wing of an airborne radar, or external to a ground radar. Still another system proposes injecting signals at the individual radiators.
Current range calibration techniques use a far field source to measure the antenna pattern at each angle off bore sight for a given pointing angle. Heretofore, it was proposed that the algorithms to re-tune the module would be derived from such current range calibration techniques. Since the implied amplitude and phase taper can be found from a fast Fourier transform of the pattern, corrections can then be applied to each module. This method is iterative and must be done for each beam position. Although noise side lobes at the -25 dBi level have been attained, the results have been inconsistent and the pattern tends to degrade quickly.
It was also proposed that the modules could be re-tuned in-flight or on the ground by measuring the antenna pattern derived from a self-contained test manifold. The pattern would be generated by phase steering the modules to each pointing angle. The FFT of this pattern would then give an implied amplitude and phase taper across the array similar to the current ground testing. However, unlike the current ground testing which takes a separate pattern for each pointing angle; this process would only produce one pattern for all pointing angles. However, with this pseudo-pattern, it is necessary to exercise the modules over many phase and amplitude settings in order to point to each pointing angle. Thus, the question arises as to which module setting should be corrected. This problem, of course, did not exist in the current ground testing because each commanded setting at each pointing angle was tuned individually. Therefore, this test manifold pattern information could restore the array patterns at each angle only if the module drift was linear across the phase and amplitude characteristics of each module. Unfortunately, the drift is not linear because the actual transfer curves of the modules change with time.
Thus, there is a need for a calibration system and method that overcomes the limitations of the proposed methods and systems as outlined above.